1. Field of the Invention
This invention relates to a guard assembly for the tip of a portable chain saw.
2. Description of the Prior Art
A well known safety problem in the use of a chain saw is "kickback". This is a rapid backward or upward motion, or both, of the chain guide bar occurring when the nose or the top area of the guide bar contacts any object such as a log or branch, or when the wood closes in and pinches the saw chain in the cut. Kickback is the greatest hazard faced by the chain saw operator, resulting in total loss of control over the saw. Tests have shown that it takes less than one tenth of a second for the chain to hinge on an obstruction and force the blade into a 45 degree rise from the horizontal.
Nose tip kickback occurs when chain running around nose of the bar strikes a rigid object. In such a case the saw swings up and back in a rotary motion, often striking the face, neck or upper body of an operator. Tests have shown that the tip of the guide bar was the point of contact in 28% of the kickback incidents observed. The cutting teeth of the chain saw drive around the guide bar at a speed equal to about 4000 feet per minute, and forces involved in kickbacks reach several hundred pounds. Thus, resulting injuries can be severe.
One way previously proposed to minimize the kickback is to install a brake on the chain saw. Unfortunately, tests have shown that it usually takes longer to stop the chain by using the brake than it takes for the kickback to occur. Even if the chain is stopped at impact, the operator would still get a powerful blow to the head. Thus brakes were found to be effective only 25% of the time, based on survey conducted by the Forest Engineering Research Institute of Canada.
This effectiveness ratio does improve with better saw maintenance and design yet it does not eliminate kickbacks completely, since it functions only after the kickback has occurred.
The prior art contains at least one bar tip guard, described in U.S. Pat. No. 4,297,786, which is the chain saw nose type guard design. This guard consists of a semicircular metal guard attached to the nose of the guide bar through the shaft and secured in position by a bolt. One of the problems with this tip guard design is the potential hazard created when the operator removes the guard for a deep cut. Once removed, the guard usually is not replaced, because it requires tools, a screwdriver at least, to install it back on the saw. There are three or four parts which must be reassembled on the chain saw bar, and the amount of time to assemble these parts also discourages replacement.
Another problem with this design is the fact that the guard attaching screw may get loose, as any screw becomes loose when subject to vibrations. The falling-off guard when hit by the high speed chain may become a dangerous projectile apt to cause injury to anyone around.
The object of the present invention is to increase the usage of nose tip guards and prevent accidental loosening and dislodgement of the guard piece.